Sealing arrangement for electrical insulating bushings



United States Patent SEALING ARRANGEMENT FOR ELECTRICAL INSULATINGBUSHINGS Donald L. Johnston, Pittsfield, and Edward F. Cronin, Lenox,Mass., assignors to General Electric Company, a corporation of New YorkApplication January 16, 1957, Serial No. 634,567

2 Claims. (Cl. 174-167) This invention relates to electrical bushings,and more in particular to an improved sealing arrangement betweenmetallic and porcelain members of a high voltage insulating bushing.

High voltage electrical insulating bushings of one type are comprised ofa central conductor extending centrally through the bushing from aterminal assembly at one end thereof. One end of the central conductoris surrounded by a ceramic insulating shell which extends from theterminal assembly to a centrally located mounting flange, the flangebeing electrically insulated from the central conductor. The presentinvention relates primarily to the seal between the terminal assemblyand the ceramic insulating member.

In one type of terminal arrangement, the central conductor is threadedon its end to receive an internally threaded terminal cap, the terminalcap also having external threads to which external connections may bemade. A frustoconical shell disc-type spring washer (hereinafterreferred to as a disc-type spring washer or spring washer), commonlyknown as a Belleville washer, was provided separated from the end of theceramic insulating member by an annular gasket, the threaded portion ofthe central conductor extending through the aperture in the disc-typespring washer. The terminal cap, upon being threaded onto the centralconductor, engaged the central portion of the spring washer, and appliedsufi'icient deflection force to the spring Washer to provide sealing atthe annular gasket on the end of the ceramic insulating member and agasket between the terminal cap and the spring washer. The spring washerwas provided to maintain forceon the gaskets while allowing fordifferent temperature coefiicients of expansion of the central conductorand ceramic shell member.

While the above described arrangement was satisfactory from thestandpoint of sealing, it was found to be undesirable in some situationssince the back pressure of the spring washer against the terminal capwas limited only by the force deflection characteristics of the springwasher. Thus, with the above described arrangement, it was possible toapply sufficient force on the spring washer to force it out of itsdesired shape. Thus, it was desirable to provide an arrangement whereinthe terminal cap could be tightened with unlimited torque as insuranceagainst loosening of the terminal cap, without adversely afiecting thespring washer.

This was accomplished by placing a rigid unthreaded disc between theceramic shell and the spring washer. The disc was supported at itsoutside diameter by the porcelain (by way of a gasket). Although thisarrangement allowed a very high torque in assembling the top terminal,the back pressure provided by the spring washer was not retained onthermal expansion of the central conductor. In other words, when thecentral conductor expanded the back pressure on the terminal cap wasprovided only by the spring washer, and thus the initial tightening ofthe terminal cap was only effective when the bushing was cool.

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It is therefore an object of this invention to provide an improved highvoltage electrical insulating bushing.

It is also an object to provide an improved seal between the ceramicinsulating member and central conductor of a high voltage electricalinsulating bushing.

It is a further object of this invention to provide a seal for a highvoltage electrical insulating bushing of the type having a disc-typespring washer extending between the ceramic insulating member and thecentral conductor, the seal being characterized by the fact that theterminal cap may be tightened without regard to the force deflectioncharacteristics of the spring washer, and the tightness is not effectedby expansion or contraction of the central conductor.

Briefly stated, in accordance with one aspect of this invention, weprovide a high voltage electrical insulating bushing comprising acentral conductor having at least one threaded end. An insulating shellis provided radially surrounding at least the portion of the centralconductor adjacent the threaded end, according to the conventionalpractice. An internally threaded terminal cap is threaded on the end ofthe central conductor. A frustoconical shell disc type spring washer isprovided extending between the threaded end of the central conductor andone end of the insulating shell, and the terminal cap is arranged tohold the spring washer in sealing engagement with the end of theinsulating shell, i.e., to provide a liquid seal between the insulatingshell and'washer. Stop means, such as a nut, is provided on the centralconductor holding the spring washer against the terminal cap, therebylimiting the deflection of the spring washer resulting from tighteningof the terminal cap on the end of the central conductor.

While the specifications conclude with claims particularly pointing outand distinctly claiming the subject matter which we regard as ourinvention, it is believed that the invention will be better understoodfrom the following disclosure taken in connection with the accompanyingdrawing.

In the drawing:

Fig. 1 is a partially cross sectional view of one form of a high voltageelectrical insulating bushing according to the invention,

Fig. 2 is an enlarged partially cross sectional view of the bushing ofFig. 1, and

Fig. 3 is a cross sectional view of a portion of the bush-' ing of Fig.1 prior to tightening of the terminal cap.

Referring now to the drawing, therein is illustrated a high voltageelectrical insulating bushing having a central conductor 10. The centralconductor may be solid, as illustrated, as is commonly the practice forcircuit breaker bushings, or it may be hollow and be provided with acentral cable as is commonly the practice with transformer bushings. Theend 11 of the bushing is threaded to engage the internal threads of aterminal cap 12, the terminal cap 12 extending axially from the upperextremity of the central conductor. An insulating shell- 13, which ispreferably of a ceramic material according to conventional practice isprovided radially surrounding the upper portion of the centralconductor, and a frustoconical shell disc type spring washer 14 extendsbetween the central conductor and the upper end 15 of the insulatingshell 13. The lower end 16 of the insulating shell 13 engages acentrally located mounting flange 17 rigidly mounted with respect to thecentral conductor and insulated therefrom.

A stop means preferably in the form of a nut 18, is also provided on theupper threaded end 11 of the central conductor. The nut 18 is positionedsuch that the spring washer 14 is rigidly held between the nut 18 andthe terminal cap 12, while sufficient axial force is exerted on thespring washer by the terminal cap to provide a 3 sealing engagementbetween the spring washer and the upper end 15 of the insulating shell.The nut 18 is preferably held firmly on the central conductor, such asby a set screw 19.

, As is commonly the practice in high voltage electrical insulatingbushings, an insulating core 20, preferably of resin-impregnated laminarinsulating material, may be provided surrounding the central conductorand extending from the lower end of the bushing, through themountingflange, and into a dielectric fluid filled chamber 21 defined bythe insulating shell 13 and central conductor 10. A ground shield 22 mayalso be provided surrounding the insulating core 20 and extending fromthe mounting flange 17 toward the lower end of the bushing.

Suitable gaskets, such as a gasket 23 between the upper end of theinsulating shell and the spring washer, a gasket 24 between the springwasher and the terminal cap, and a gasket 25 between the lower end ofthe insulating shell and the mounting flange, are provided to ensuresealing of the dielectric fluid filled chamber 21. A gasketed plug 26 isprovided in the spring washer to facilitate filling of the chamber 21with dielectric fiuid.

In the fabrication of the upper portion of the bushing (see Fig. 3), thestop nut 18 is first threaded onto the end of the central conductor fora predetermined distance, depending upon the length of the insulatingshell. It is preferable that the nut be located such that duringoperation of the bushing the condition never arises when the springwasher is dished in the opposite direction from its normal position, thespring washer being initially assembled with its convex surface towardthe upper end of the bushing. In order to ensure that the abovecondition does not occur as a result of thermal contraction of thecomponents of the bushing, a reference surface or line may be providedon the nut, such as the shelf 28 provided below the upper surface of thenut 18, the reference being initially aligned with a reference on theinsulating shell, such as the upper end 15, during assembly of thebushing at a predetermined temperature, such as room temperature. Thereferences should be located so that the opposite dishing of the washerwill not occur when the bushing is subject to the extreme temperaturesnormally expected during operation of the bushing, and thus, asillustrated in the drawing, the shelf 28 is located within the plane ofthe upper end 15 of the insulating shell, so that a portion of the nutextends upwardly beyond the upper end 15 of the insulating shell. Whilethe disc type spring washer is illustrated as being flat in Figs. 1 and2 for the sake of clarity of the drawing, in actual operation of thebushing the center of the washer would be slightly dished toward theupper end of the bushing.

Still referring to Fig. 3, after the nut 18 is in place and secured withthe set screw 19, the spring washer is placed over the centralconductor, with the convex surface toward the upper end of the bushing.Then the terminal cap is threaded over the end of the central conductor,and provided with sufiicient torque that the lower face of the springwasher tightly engages the upper surface of the nut 18, as illustratedin Figs. 1 and 2. At this time, sealing engagement will occur betweenthe outer lower surface of the spring washer and the upper end of theinsulating shell. 7

The arrangement of this invention, as previously disclosed, provides abushing in which the back pressure on the terminal cap is not limited bythe force deflection characteristics, as in the case of previousbushings of this type not supplied with a stop nut, and thereforesubstantially unlimited torque may be applied to the terminal cap. Thisis extremely advantageous from the standpoint that personnel assemblingpreivous bushings in some cases applied too much torque to the terminalcap and effective sealing of the bushing was lost because of dishing ofthe spring washer in the opposite direction (i.e.,. toward the lower endof the bushing), and the force defiection characteristics of the washervaried due to deflection beyond the elastic limit of the material.

The arrangement of this invention also has the advantage that the backpressure on the terminal cap is not lost as a result of thermalcontraction or expansion of the bushing, as in the case Where anon-threaded disc is placed under the spring washer and bears againstthe end of the insulating shell, since the spring washer in thearrangement of the present invention is held tightly between theterminal cap and the stop nut.

The stop nut also acts to gauge the deflection of the spring washer, andthus enables the obtaining of maximum sealing benefit from the forcedeflection characteristics of the spring washer.

It will be understood, of course, that, while the form of the inventionherein shown and described constitutes the preferred embodiment of theinvention, it is not intended herein to illustrate all of the possibleequivalent forms or ramifications thereof. It will also be understoodthat the words employed are words of description rather than oflimitation, and that various changes may be made without departing fromthe spirit or scope of the invention herein disclosed. For example,while the bushing has been described interms of upper and lower parts,these terms are purely relative, and do not indicate the physicalposition of the bushing in operation. It is aimed in the appended claimsto cover all such changes as fall within the true spirit and scope ofthe invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a high voltage electrical insulating bushing, a central conductorhaving a threaded end, dished spring washer means on said threaded end,an insulative shell radially surrounding at least a portion of saidcentral conductor, said washer means being supported at its periphery byone end of said insulating shell, terminal cap means on the threaded endof said central conductor contacting said washer means on one sidethereof for causing flexing of said Washer means and for holding saidwasher means in sealing engagement with said one end of said insulatingshell, stop means on said central conductor on the side of said washermeans opposite said one side thereof for limiting the axial movement ofsaid washer means with respect to said central conductor, said stopmeans having a washer contacting surface and a shelf portion lying in aplane spaced from said washer contacting surface a predetermineddistance, said shelf portion being adapted to serve as a reference planefor aligning 'said stop means with the washer supporting end of saidinsulating shell, and said'shelf portion being located in the sameradial plane with respect to said central conductor as said one end ofthe insulating shell, so that the washer contacting surface'extendsbeyond the insulating shell a distance equal to said predetermineddistance, and said predetermined distance being such that said washercontacting surface when spaced beyond said insulating shell will act asa stop preventing said washer from being flexed beyond its elasticlimit, whereby substantially unlimited torque can be applied to saidterminal cap means in assembling the parts of the bushing withoutcausing opposite dishing of said washer.

2. The bushing of claim 1 in which said stop means is a nut threaded onsaid central conductor.

References Cited in the file of this patent UNITED STATES PATENTS1,877,586 Rah Sept. 13, 1932 FOREIGN PATENTS 320,953 Great Britain Oct.28, 1929 OTHER REFERENCES Publication: R. N. Anderson et al., NewApparatus Bushing With Improved Characteristics, Transactions ofA.I.E.E., vol. 71, part III, August 1952, pages 628-630.

